The invention relates to multistage x-ray image intensifiers according to the preamble of claim 1. Image intensifiers of this type are, for example, known from the U.S. Pat. No. 2,555,423.
In the case of multistage image intensifiers, in which the x-ray image is first converted into an electron image, the individual stages intensifying the electron image through acceleration in the electrical field, respectively, are coupled with one another via screens which exhibit, as significant elements, a fluorescent or cathodoluminescent layer and a photocathode layer which is optically in contact therewith. However, these two reactive layers per se usually have only little stability (or strength). Therefore, they are applied on opposite sides of a sufficiently stable carrier plate which allows the light released in the fluorescent layer to pass through to the photocathode layer applied on the other side.
The carrier must be so constructed that a good "contact copy" of the fluorescent screen image results on the photocathode; i.e., that the carrier effects no reduction in the definition (or sharpness) of the image. This is attainable e.g. through the utilixation of fiber optics plates as carriers for the layers. However, they have the major disadvantage that they are very expensive.
A reduction in costs could be achieved through the utilization of transparent plates or foils; for example consisting of glass, mica, or a vacuum-stable organic material (e.g. polyimide), or the like. However, the thickness of the carrier layer cannot be permitted to effect any substantial scattering of the traversing light. For the definitions, utilizable as a rule in these devices, a thickness of the carrier layer e.g. up to fifty microns (50 .mu.m) would still be permissible. In the case of known methods, first the fluorescent layer is applied on the carrier. These subsequently follows the incorporation of the carrier and fluoroscent layer in the image intensifier arrangement and finally the application of the photocathode layer on the still free surface of the carrier which lies opposite the fluorescent layer.
Most the intermediate screens, manufactured in a known fashion, exhibit non-uniform brightness transmission over the surface. This can be explained, for example, in that even in the case of originally satisfactorily clean carrier surfaces, with the deposition of the fluorescent material on the one side of the carrier, the opposite side, to be coated later with the photocathode, becomes "contaminated" by the means employed during the manufacture of the fluorescent screen (because, on account of the sensitivity of the thin foil, no satisfactory protection is possible). The low stability of the foil is also the reason that the "contaminated" surface cannot subsequently be satisfactorily cleansed. Thus the local defects (or disturbances) of the photocathode, and hence a patchy (or spotty) image is obtained.